This invention relates generally to the field of thermal barrier coatings, and more particularly to a thermal barrier coating for very high temperature applications, such as in a combustion turbine engine. In particular, this invention relates to the field of ceramic thermal barrier coatings having high phase stability at 1400xc2x0 C. and higher, which are resistant to sintering damage, for coating superalloy or ceramic components in the hot sections of a combustion turbine, such as turbine blades and vanes, transitions, ring segments and combustors.
The demand for continued improvement in the efficiency of combustion turbine and combined cycle power plants has driven the designers of these systems to specify increasingly higher turbine inlet temperatures. Although nickel and cobalt based superalloy materials are now used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures sometimes as high as 1400xc2x0 C.
It is known in the art to coat a superalloy metal component with an insulating ceramic material to improve its ability to survive high operating temperatures, for example U.S. Pat. No. 4,321,310 (Ulion et al). It is also known to coat the insulating ceramic material with an erosion resistant material to reduce its susceptibility to wear caused by the impact of particles carried within the hot gas flow path; for example, U.S. Pat. Nos. 5,683,825 and 5,562,998 (Bruce, et al. and Strangman, respectively).
Much of the development in this field of technology has been driven by the aircraft engine industry, where turbine engines are required to operate at high temperatures, and are also subjected to frequent temperature transients as the power level of the engine is varied. A combustion turbine engine installed in a land-based power generating plant is also subjected to high operating temperatures and temperature transients, but it may also be required to operate at full power and at its highest temperatures for very long periods of time, such as for days or even weeks at a time. Prior art insulating systems are susceptible to degradation under such conditions at the elevated temperatures demanded in the most modern combustion turbine systems.
U.S. Ser. No. 09/245,262, filed on Feb. 2, 1999 (Subramanian, et al.; ESCM 283139-00491), also related to columnar thermal barrier coatings (TBCs), usually of yttria-stabilized zirconia (YSZ), deposited by electron beam physical vapor deposition (EB-PVD) with a sintering resistant layer of aluminum oxide or yttrium aluminum oxide, deposited as a continuous or discontinuous layer between submicron gaps in the TBC columns. This material was thermally stable up to about 1200xc2x0 C. Other columnar TBC coatings are described in U.S. Ser. No. 09/393,415, filed on Sep. 10, 1999, (Subramanian; ESCM 283139-00224), where TBC columns had a composition of (A,B)xOy and were covered by a sheath of a composition of CzOw, where A,B and C were selected from Al, Ca, Mg, Zr, Y, Sc and rare earth equal to La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. In this application, a reaction between CzOw and (A,B)xOy was key to obtain a multiphase TBC system which was expected to be sinter resistant and strain tolerant up to 1400xc2x0 and higher. The same materials were used as an (A,B)xOy planar based TBC coated with a CzOw overlay in U.S. Ser. No. 09/393,417, filed on Sep. 10, 1999, (Subramanian; ESCM 283139-00223). In this application also, a reaction between CzOw and (A,B)xOy was key to obtain a multiphase TBC system which was expected to be sinter resistant and strain tolerant up to 1400xc2x0 and higher. Specific compounds capable for application as TBCs are described in U.S. Ser. No. 09/405,498, filed on Sep. 24, 1999 (Subramanian, et al.; ESCM 283139-00076). There, TBC layers of LaAlO3, NdAlO3, La2Hf2O7, Dy3Al5O12, Ho3Al5O12, ErAlO3, GdAlO3, Yb2Ti2O7, LaYbO3, Gd2Hf2O7, and Y3Al5O12 were generally described. These were compounds capable for TBC application, due to their inherently superior sintering resistance and phase stability.
A solid, vapor deposition material useful for the EB-PVD method to provide heat resistant coatings in aircraft engines and the like, where excellent heat resistance and thermal shock resistance is required, is taught by U.S. Pat. No. 5,789,330 (Kondo, et al). There, the material is sintered zirconia, containing a special stabilizer selected from yttria, magnesium oxide, calcium oxide, scandium oxide, or oxides of rare earth elements equal to La, Ce, Pr, Nd, Pm, Sm, Eu, Ed, Tb, Dy, fermium, Wr, thulium, Yb and ruthenium in the range of 0.1 wt percent to 40 wt percent of the material. The sintered material has 25% to 70% monoclinic phase and up to 3% tetragonal phase, with the rest as cubic phase.
Some high temperature resistant coatings, as taught in U.S. Pat. No. 5,304,519 Jackson, et al), have utilized thermal spraying of zircon plus zirconia particles (ZrSiO4 and ZrO2 respectively) partially stabilized with an oxide selected from CaO, Y2O3, MgO CeO2, HfO2 or rare earth oxide, where rare earth equal La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. These materials are used as refractory, thermal shock resistant coatings for hearth rolls for annealing steel, stainless steel and silicon steel sheet at furnace temperatures between 820xc2x0 C. and 1100xc2x0 C.
Data regarding sintering rates of single oxides AxOy are available but only a few publications discuss sintering rates of multicomponent oxides. One such publication is by Shinozaki, et al. 1981, (9), pp. 1454-1461 where the sintering tendencies of a solid solution of mixed Sm2O3xe2x80x94ZrO2 were discussed in The Chemical Society of Japan, xe2x80x9cSintering Sm2O3xe2x80x94ZrO2 Solid Solution.xe2x80x9d There, tablets of the mixed component oxides at various mole % were sintered at from 1200xc2x0 C. to 1600xc2x0 C. and isothermal linear shrinkage was measured, The least amount of sintering, 3% to 10% at 1400xc2x0 C., was found at ranges of 5 mole % to 50 mole % Sm2O3.
In xe2x80x9cLa2Zr2O7xe2x80x94a new candidate for thermal barrier coatingsxe2x80x9d, R. Vaxcex2en, X. Cao, F. Tietz, G. Kerkhoff, D. Stxc3x6ver, United Thermal Spray Conference, 17.-19.3.99, Dxc3xcsseldorf, Hrsg. E. Lugscheider, P. A. Kammer, Verlag Fxc3xcr Schweixcex2en und Verwandte Verfahren, Dxc3xcsseldorf, 1999, p. 830-034, plasma sprayed TBC coatings of one specific compound, La2Zr2O7, were discussed. Although this material is of the pyrochlore structure, as shown in their FIG. 2, our own results in the Example, below, show this specific compound is not good as a TBC. However, introduction of cation excess/defects or oxygen defects change the sintering properties and this is not suggested in the paper.
What is needed is a TBC coating for a device, where the coating will remain thermally stable, protective, strain compliant, and resistant to substantial sintering of gaps in its grain structure, for use in long-term, high temperature turbine applications at temperatures up to 1400xc2x0 C. Preferably the TBC will be a new material which itself meets the above criteria without the need for extra processing steps or additional coating.
It is a main object of this invention to provide a device which is capable of operating at temperatures up to about 1400xc2x0 C. for extended periods of time with reduced component degradation. It is a further object of this invention to provide a method of producing such a device that utilizes commercially available materials processing steps.
These and other objects of the invention are accomplished by providing a device for operating over a range of temperatures, having a deposited thermal barrier coating on at least a portion of its surface, the device comprising a substrate with a bond coat; and then a deposited ceramic thermal barrier layer, the ceramic layer consisting essentially of a pyrochlore crystal structure having a chemical formula consisting essentially of An+2xe2x88x92xBm+2+xO7xe2x88x92y, where A is selected from the group of elements consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and mixtures thereof; where B is selected from the group of elements consisting of Zr, Hf, Ti, and mixtures thereof; n and m are the valence of A and B respectively; and for xe2x88x920.5xe2x89xa7xxe2x89xa70.5, y=7xe2x88x92([(2xe2x88x92x)n+(2+x)m]/2) or y=7xe2x80x94(((2xe2x88x92x)n+(2+x)m)/2), that is:       y    =          7      -                        [                                                    (                                  2                  -                  x                                )                            ⁢              n                        +                                          (                                  2                  +                  x                                )                            ⁢              m                                ]                2              ,
and excluding the following combinations for x=0, y=0: A=La and B=Zr; A=La and B=Hf; A=Gd and B=Hf; and A=Yb and B=Ti, which describe the following excluded compounds: La2Zr2O7, La2Hf2O7, Gd2Hf2O7, and Yb2Ti2O7. The preferred combinations for this invention are A=Sm and B=Zr; A=Eu and B=Zr; A=Gd and B=Zr; with the first combination being the most preferred.
Further, a method according to this invention, for producing a device operable over a range of temperatures, includes the steps of: providing a substrate; depositing a bond coat and then depositing a ceramic thermal barrier layer over the bond coat in a manner that provides a deposited ceramic layer consisting essentially of a pyrochlore crystal structure having a chemical formula consisting essentially of An+2xe2x88x92xBm+2+xO7xe2x88x92y, where A is selected from the group of elements consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and mixtures thereof; where B is selected from the group of elements consisting of Zr, Hf, Ti and mixtures thereof; n and m are the valence of A and B respectively; and for xe2x88x920.5xe2x89xa7xxe2x89xa70.5,       y    =          7      -                        [                                                    (                                  2                  -                  x                                )                            ⁢              n                        +                                          (                                  2                  +                  x                                )                            ⁢              m                                ]                2              ,
and excluding the following combinations for x=0, y=0: A=La and B=Zr; A=La and B=Hf; A =Gd and B=Hf; and A=Yb and B=Ti.
These compositions will be extremely stable even under long term exposure to temperatures up to about 1500xc2x0 C. and can be deposited by well known plasma spray, EB-PVD, and D-gun techniques, HVOF (high velocity oxygen fuel deposition) techniques, inductively coupled deposition processes, and electron beam directed vapor deposition techniques.